A wide variety of varicap diode structures have been described. Certain of these structures have been designed or adapted for incorporation in integrated circuits. The use of such structures in semi-custom integrated circuits has however presented considerable difficulty. In a semi-custom circuit the manufacturer provides a basic circuit structure, e.g. an array of similar device blocks or cells, which is then interconnected in a particular way by one or more customer-specified masks to provide a desired circuit function. Clearly, it is to the manufacturer's advantage if he can provide a versatile basic circuit structure with a wide variety of applications so as to minimise his production costs. In such circuits, varicap diodes may be required by some customers but not by others. Thus, if conventional varicap diode structures are incorporated in a semi- custom circuit, these structures may occupy chip space that for some customers is effectively redundant. Moreover, conventional diode structures are generally fixed in maximum and minimum capacitance value prior to the application of the customising interconnections and are not amendable to modification to suit individual customer requirements.